If there was a moment when, symbolically at least, last season's title race tipped decisively the way of Chelsea, it came 12 minutes after half-time of their game at Manchester City in December. City were leading 1-0 when Kevin De Bruyne, four yards out and with the goal gaping, slammed a Jesús Navas cross against the bar. Within three minutes Chelsea were level through Diego Costa and, as City lost their heads, Antonio Conte's side went on to win 3-1.

If there was a moment when, symbolically at least, last season's title race tipped decisively the way of Chelsea, it came 12 minutes after half-time of their game at Manchester City in December. City were leading 1-0 when Kevin De Bruyne, four yards out and with the goal gaping, slammed a Jesús Navas cross against the bar. Within three minutes Chelsea were level through Diego Costa and, as City lost their heads, Antonio Conte's side went on to win 3-1.

That game was 15 months ago, yet it may as well have been a lifetime. It is not so much that City have improved, although they clearly have, as that such a performance from Chelsea seems implausible now.

That fight, that aggression, that ruthlessness is gone and, not coincidentally, so too is Costa, the embodiment of those qualities.

Conte is by no means the first Chelsea manager to chafe against the club's transfer policy but a not insignificant part of their problems this season has been provoked by the breakdown of his relationship with the centre-forward. In that regard at least he cannot blame the board: he forced Costa's departure with the text message he sent him and was then given a £60m replacement in Álvaro Morata and, when the new striker lapsed into an odd insipidness, Conte was given another forward, the £18m Olivier Giroud.

The manager also bears culpability for a tactical shift he has made that seems to have had little positive impact, although he may argue that he was forced into the move by the inadequacy of the replacements he was given for Nemanja Matic, whose transfer to Manchester United, as well as whether Conte opposed it, remains a source of controversy.

Last season Chelsea became the first side since Everton in 1962-63 to win the league playing predominantly with a back three. The use of the third defender along with two holding midfielders - one of whom was a highly effective passer of the ball, Matic registering seven assists last season - provided a solid base that liberated the wing-backs, who could link up with the two inside-forwards, Eden Hazard and Willian or, more usually, Pedro.

It was those creators who were key, because they operated in areas that were very difficult for opponents to pick up, outside the holding midfielder(s), too deep for the central defenders and too narrow for the full-backs, who were concerned anyway by the threat of the wing-backs. In the crowded muddle of a modern Premier League midfield, Conte had found previously unrecognised space (or at least space that had been largely neglected since back fours came into vogue in the early '60s).

So effective was the ploy that the only way opponents seemed able to combat it was to mirror the shape, so that two holders became in effect wing-halves picking up inside-forwards as they had done in the old days, the pressure to hold a position in front of the defensive line to prevent a runner from deep being able to attack a flat line alleviated by the fact that one of three central defenders could step up to fill that space.

The 3-4-2-1 was an oddity in that it was best combated by itself, making it a dead end in terms of tactical evolution, but it was hard to see any other obvious countermeasure.

Yet this season it has faded away, in part because Conte, particularly in big games, has chosen to push one of those inside forwards deeper, making a 3-5-1-1. That places an enormous creative burden on Hazard, a burden made even more onerous than it would have been anyway by the fact that he has not had a consistently firing centre-forward to play off.

It probably makes sense to evolve from a position of strength, and playing those three holders worked for Conte - eventually and not entirely convincingly - at Wembley in August in the win over Tottenham , perhaps the team who had best replicated the 3-4-2-1 shape.

But the widespread use of the system may suggest a concern about the quality of N'Golo Kanté's partner at the back of midfield. Neither Tiémoué Bakayoko nor Danny Drinkwater has really settled, meaning Cesc Fàbregas is often forced into service alongside Kanté, something that was clearly not the plan last season. In the circumstances, bolstering that area makes sense for Conte.

That said, Fàbregas played alongside Kanté at the Etihad last season, his passing one of the reasons for Chelsea's excellence on the counter, and played perfectly well in the role against Barcelona last week. With Willian in superb form, and no great reason to trust Bakayoko, who may be injured, or Drinkwater, it may be that Conte opts again for the 3-4-2-1 today.

It is not just that Chelsea have changed, though; so too have City. And this time it is Pep Guardiola's side who, in De Bruyne and David Silva, have the two in-form creators operating in what are to all intents inside-forward roles that other sides have desperately struggled to suppress.